Photostabilization of Polyamides

A number of organic and inorganic stabilizers are recommended for increasing light stability. 

As the inorganic additives to polyamides recommended in the literature, we should mention primarily salts of trivalent chromium [77, 78). The use of compounds of di- and hexavalent chromium is also possible. However, during the process of treatment of the polyamide with the stabilizer, divalent chromium is oxidized, while hexavalent chromium is reduced to the trivalent form. Thus, for example, polyamide fibers are treated first with potassium bichromate and then with sodium thiosulfate [77]. Certain chromium dyes also exhibit a protective action against sunlight [79]. 

Light-stable polyamides are obtained by adding an aqueous suspension of carbon black containing water-soluble chromium salts, for example, CrF3, before polycondensation [78]. In addition to increasing the photostability of polyamides, chromium also increases their stability to thermal oxidation [68, 41]. The spectra of polyamide samples treated with chromium, cited in Fig. 139, practically do not differ from the spectra of the imtreated samples. The action of chromium apparently reduces to a strengthening of the amide bond, which is important both in photoaging and in thermal oxidation of polyamides [68]. 

Inorganic additives cerium hydroxide or oxide [41], compounds of pentavalent niobium and tantalum [84], are also used as photostabilizers. 

The absence of a protective action of an additive can be explained by its ability to give rays upon luminescence with a greater wavelei th than those absorbed, however, possessing energy sufficient for the cleavage of weak bonds in the polymer chain (for example, the amide bond). 

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